Two New Neptunyl(V) Selenites: A Novel Cation–Cation Interaction Framework in (NpO₂)₃(OH)(SeO₃)(H₂O)₂·H₂O and a Uranophane-Type Sheet in Na(NpO₂)(SeO₃)(H₂O)

Dark green crystals of (NpO2)3(OH)(SeO3)(H2O)2·H2O (1) have been prepared by a hydrothermal reaction of neptunyl(V) and Na2SeO4 in an aqueous solution at 150 °C, while green plates of Na(NpO2)(SeO3)(H2O) (2) have been synthesized by evaporation of a solution of neptunyl(V), H2SeO4, and NaOH at room temperature. Both compounds have been characterized by single-crystal X-ray diffraction. The structure of compound contains three crystallographically unique Np atoms that are bonded to two O atoms to form a nearly linear ONpO NpO2+ cation. Neighboring Np5+ ions connect to each other through a bridging oxo ion from the neptunyl unit, a configuration known as cation–cation interactions (CCIs), to build a complex three-dimensional network. More specifically, each Np(1)O2+, Np(2)O2+, and Np(3)O2+ cation is involved in three, five, and four CCIs with other units, respectively. The framework of neptunyl(V) pentagonal bipyramids is decorated by selenite trigonal pyramids with one-dimensional open channels where uncoordinated waters are trapped via hydrogen bonding interactions. Compound adopts uranophane-type [(NpO2)(SeO3)]− layers, which are separated by Na+ cations and water molecules. Within each layer, neptunyl(V) pentagonal bipyramids share equatorial edges with each other to form a single chain that is further connected by both monodentate and bidentate selenite trigonal pyramids. Crystallographic data: compound, monoclinic, P21/c, Z = 4, a = 6.6363(8) Å, b = 15.440(2) Å, c = 11.583(1) Å, β = 103.549(1)°, V = 1153.8(2) Å3, R(F) = 0.0387 for I > 2σ(I); compound (2), monoclinic, C2/m, Z = 4, a = 14.874(4) Å, b = 7.271(2) Å, c = 6.758(2) Å, β = 112.005(4)°, V = 677.7(3) Å3, R(F) = 0.0477 for I > 2σ(I).